A circuit of this length does not occur in the normal myocardium, and for this reason, spontaneous ventricular re-entry in the normal heart is very rare. However, if conduction velocity is slowed by 50%, to 0.5 m/sec, and the refractory period is shortened by 50%, to 150 ms, then the length of the circuit needed to permit re-entry will shorten to 7.5 cm or approximately 3 inches (0.5 m/sec x 0.15 sec =0.075 m). Greater slowing of conduction or greater shortening of the refractory period will further shorten the length of the circuit required to permit re-entry.